US president Barack Obama, under mounting pressure to take action if the chemical weapons claims are proven, has also been meeting with his national security advisers to discuss reports of the attack.

US secretary of state John Kerry spoke to Syrian foreign minister Walid al-Moualem and told him the Damascus government should have let UN inspectors have access to the site of an alleged gas attack, the US state department said.

A state department official said Mr Kerry called "to make clear" that access should have been granted "if, as they claimed, the Syrian regime has nothing to hide".

"It should have allowed immediate and unimpeded access to the site rather than continuing to attack the affected area to block access and destroy evidence," the official said.

"The secretary further emphasised ... that he had received full assurances from Free Syrian Army commanders that they would ensure the safety of UN investigators into the targeted areas."

The Syrian government has strongly denied the allegations but has yet to accede to demands that UN inspectors already in the country be allowed to visit the sites of the alleged attacks.

However, Syrian information minister Omran al-Zohbi says the regime has never used chemical weapons.

"We have never used chemical weapons in Syria, in any form whatsoever, be it liquid or gas," he told a Beirut-based Arabic television channel.

However, Syrian foreign minister Walid Muallem was quoted by his Iranian counterpart, Mohammad Javad Zarif, as saying Damascus would facilitate a visit of UN inspectors to the scene of the alleged attack.

"The Syrian government will cooperate with the United Nations mission now in Syria to create the conditions for a visit to zones where terrorist groups have carried out attacks with chemical weapons," he was quoted as saying.

"We are currently in the process of discussions with the United Nations mission on preparing this visit."

On Friday, the National Coalition pledged to guarantee the safety of the inspectors but warned that the "clock is ticking" before alleged evidence vanishes.

Meanwhile, Doctors Without Borders (MSF) says around 3,600 patients displaying "neurotoxic symptoms" had flooded into three Syrian hospitals on the day of the alleged attacks and 355 of them died.

The victims all arrived within less than three hours of each other, and MSF director of operations Bart Janssens said the pattern of events and the reported symptoms "strongly indicate mass exposure to a neurotoxic agent".

"Medical staff working in these facilities provided detailed information to MSF doctors regarding large numbers of patients arriving with symptoms including convulsions, excess saliva, pinpoint pupils, blurred vision and respiratory distress," he said.

But MSF stressed it had no scientific proof of the cause of the symptoms nor could it confirm who carried out the attack.

For its part, the Syrian Observatory for Human Rights said more than 300 people had died from the effects of gas, including 82 women and 54 children.

Rudd says Assad regime has to establish 'absence of culpability'

Last night Mr Rudd interrupted his election campaign to attend a national security briefing on the situation in Syria.

He had been scheduled to spend Saturday night in Brisbane, but diverted his campaign for the briefing with officials, including Foreign Minister Bob Carr and Minister for Defence Materiel, Mike Kelly.

"It is too horrible to believe that in the year 2013 we could see civilians, men, women and children, being slaughtered by what appear to be chemical weapons," he said this morning.

"The Government's assessment is that the indications point strongly in the direction of the use of chemical weapons and indications also point in the direction of the Syrian regime.

"However, definitive conclusions on this await final and full access to the site by a UN weapons inspectors.

"The burden of proof now lies with the Syrian regime to establish their culpability or absence of culpability on this matter."

Mr Rudd said the effort of the UN Security Council is now focused on encouraging Syria to allow weapons inspectors to visit the site in Damascus.

"We will be working with all members of the Security Council ... to forge the consensus necessary to ensure that ... if it is concluded that the Syrian regime is responsible for a chemical weapons attack, that we would then act in concert with other countries to determine an appropriate set of responses," he said.

"I think it's unproductive and unwise to begin to speculate on any form of action and what shape that may take the business of responding to international crises as this is emerging as one is to take it calmly and methodically step by step."

International community under mounting pressure to act

The White House says Mr Obama on Saturday received a "detailed review of a range of potential options" from his top advisers on how the United States and its allies could respond to an alleged chemical weapons attack.

Mr Obama, who is under pressure to act, also spoke with British prime minister David Cameron about Syria and agreed to consult about "potential responses by the international community," the White House said.

A spokesman for Mr Cameron sais the leaders agreed on the need to deter the use of chemical weapons and they were concerned about "increasing signs" that Syria had attacked civilians.

"They are both gravely concerned by the attack that took place in Damascus on Wednesday and the increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people," the spokesperson said.

"They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options.

"The fact that president (Bashar al-)Assad has failed to cooperate with the UN suggests that the regime has something to hide."

French foreign minister Laurent Fabius, during a visit to the West Bank on Saturday, blamed Syria for a "chemical massacre" and said "the Bashar regime is responsible".

Russia is urging Damascus to cooperate with the UN but has dismissed calls for use of force against its ally.

Syrian ally Iran blamed the rebels and warned the West against any military intervention.

Warning against any Western military intervention in the conflict, Mr Araqchi said "there is no international authorisation for" such action.

The UN says more than 100,000 people have been killed in Syria since an uprising against president Bashar al-Assad's rule flared in March 2011, while millions more have fled the country or been internally displaced.

Mr Ban is determined to "conduct a thorough, impartial and prompt investigation" into the chemical attack claims, his spokesman said.